1. Technical Field
The present invention is directed to a system for capturing images of patterned objects. In particular, the present invention includes a treatment for an imaging surface of a fingerprint image capturing system to provide high contrast images of the fingerprint.
2. Related Art
Patterned object recognition systems are becoming common in industrial and commercial settings and have a variety of uses. Recently, manufacturers have been attempting to reduce costs associated with pattern recognition systems to make them more viable for consumer use. One such consumer application for pattern recognition systems includes fingerprint image acquisition and recognition. Such a system is useful, for example, to enhance computer security by reading a potential user's fingerprint to compare with the fingerprints of users authorized to use a computer or access certain files or functions of a computer. Such a system could, for example, take the place of a security system that uses a login name and password.
The first thing such a fingerprint recognition system, or any pattern recognition system, must be able to do is to accurately acquire the fingerprint, or other pattern, for analysis. A number of mechanisms exist for such acquisition of pattern data. For example, U.S. Pat. Nos. 3,975,711; 4,681,435; 5,051,576; 5,177,435 and 5,233,404 all disclose apparatuses for acquiring an image of a patterned object.
FIG. 1 shows a schematic diagram of one such prior art optical fingerprint capturing and recognition system. In FIG. 1, an image capturing system 108 includes a light source 112, an optical prism 110, and a light detector 114. The prism 110 includes an imaging surface 118, a light receiving surface 122, and a viewing surface 120. Imaging surface 118 is the surface against which a patterned object, such as a fingerprint, is placed for imaging. The light source 112, which may, for example, include one or more light emitting diodes (LEDs), is placed adjacent to light receiving surface 122 and generates incident light 124 that is transmitted to the optical prism 110.
Generally, incident light 124 strikes imaging surface 118 and can either pass through, reflect from, or scatter off of imaging surface 118 to form an image of fingerprint 135. Reflected and/or scattered light 130 passes out of viewing surface 120 of prism 110 and through light detector 114 located adjacent to viewing surface 120. Light detector 114 may contain one or more optical lenses 115 and an image sensor 116 for capturing optical light images and converting them into electrical signals. Such an image sensor 116 could include, for example, a charge coupled device (CCD) or complementary metal oxide semiconductor (CMOS) chip. The electrical signals can be processed by processing electronics and/or software (not shown) and stored in a form to allow comparison with other fingerprint images.
As shown in FIG. 1, fingerprint 135 has both valleys 109 and ridges 111. There are at least two methods by which an image capturing system such as system 108 can image valleys 109 and ridges 111 of fingerprint 135. Both methods rely on a fingerprint ridge 111 being in contact with imaging surface 118 and a fingerprint valley 109 being above and out of contact with imaging surface 118. In a first method, known as an "absorption" method, incident light 124 which strikes a region of imaging surface 118 where a fingerprint ridge 111 is in contact therewith is substantially absorbed or scattered by ridge 111. Incident light 124 which strikes a region of imaging surface 118 where there is a fingerprint valley 109, however, is totally internally reflected from imaging surface 118 to enter light detector 114. A portion of the absorbed and/or scattered light from fingerprint ridges 111, however, is not picked up by light detector 114. Accordingly, in an absorption system, the image of fingerprint 135 is relatively bright at fingerprint valleys 109 and relatively dark at fingerprint ridges 111.
In a second method, known as a scattering method, incident light 124 is projected onto imaging surface 118 such that in regions of a fingerprint ridge 111, incident light 124 is scattered into prism 110 and in regions of a fingerprint valley 109, incident light 124 substantially passes through imaging surface 118. Light detector 114 picks up the scattered light and does not pick up the light which passes through imaging surface 118. Thus, in a scattering system, the image of a fingerprint 135 is relatively dark at fingerprint valleys 109 and relatively bright at fingerprint ridges 111.
In either a scattering system or an absorption system, the ability of an image capture system to create a high contrast, accurate fingerprint image is dependent upon the fingerprint ridges 111 touching the imaging surface 118 and fingerprint valleys 109 not touching imaging surface 118. Specifically, in an absorption system, it is the contact between the fingerprint ridge 111 and imaging surface 118 which causes the absorption and/or scattering of incident light 124 and the lack of contact between a fingerprint valley 109 and the imaging surface 118 which allow total internal reflection of incident light 124 to create an image of the fingerprint 135. In a scattering system, it is the contact between a fingerprint ridge 111 and imaging surface 118 which causes incident light 124 to be scattered into lens assembly 114 and the lack of contact between a fingerprint valley 109 and imaging surface 118 which allows incident light 124 to pass through imaging surface 118.
Accordingly, to generate an accurate image of a fingerprint 135, fingerprint ridges 111 should have relatively complete contact with imaging surface 118.
However, at least two difficulties can arise in connection with establishing relatively complete contact between fingerprint ridges and imaging surface 118. First, if prism 110 is formed from glass, imaging surface 118 will have been ground by an abrasive substance to smooth imaging surface 118. While this grinding process leaves imaging surface 118 relatively smooth on a macroscopic scale, as shown in FIG. 2, which is an enlarged side schematic view of imaging surface 118, the abrasive substance used to grind imaging surface 118 can form indentations or pits 119 in imaging surface 118. A fingerprint ridge 111 will generally not fill in a pit 119 when fingerprint 135 is placed against imaging surface 118. Accordingly, pits 119 cause an area of non-contact between imaging surface 118 and fingerprint ridge 111. For the reasons explained above, this can cause deterioration of a fingerprint image generated by image capturing system 108.
A second difficulty in establishing relatively complete contact between a fingerprint ridge 111 and imaging surface 118 is also illustrated in FIG. 2. As shown, the ridges 111 of fingerprint 135 are generally uneven and have gaps or irregularities 117. Therefore, fingerprint ridges 111 may not make complete contact with imaging surface 118. And, the irregularities 117 of fingerprint ridges 111 which are not actually touching imaging surface 118 will not, or will only partially, scatter or absorb incident light 124. This can cause a blurry or un-clear fingerprint image and/or can reduce the contrast of a fingerprint image.
Prior art fingerprint readers have addressed these problems in various ways. For example, U.S. Pat. No. 5,737,071 issued to Arndt on Apr. 7 1998 ("Arndt") is directed to a method for enhancing live-scan fingerprint reader images. Arndt discloses providing an absorbent pad containing chemicals to coat the surface of a person's finger whose fingerprint is to be taken. After being coated with chemicals from the pad, the person's finger is placed against a window or platen of the fingerprint reader to allow an image to be taken. The chemicals from the pad tend to fill in the regions of the fingerprint ridge/platen interface where the fingerprint ridge does not quite touch the platen. The chemical which fills in these regions can act as a fingerprint ridge which is fully touching the platen to scatter and/or absorb the incident light. However, not enough chemical is retained on the fingerprint to fill in the fingerprint valleys. Thus, incident light will still totally internally reflect from (absorption system) or pass through (scattering system) the platen at fingerprint valleys. In this way, a higher contrast, sharper fingerprint image can be obtained.
However, there are some difficulties with the method and apparatus disclosed in Arndt. First, the user of the fingerprint reader must use a separate absorbent pad prior to using the scanner. This can make use of such a system inconvenient for use as a PC security system. Additionally, chemical residue can be left on the platen after a fingerprint image is captured. This chemical residue can interfere with subsequent capturing of fingerprint images and collect debris which can further interfere with capturing of fingerprint images. Further, if substantial enough, such chemical residue can make it appear as though a fingerprint is still on the window. Thus, the fingerprint ready may be able to be "tricked" into reading a false match of a fingerprint, though there is no actual fingerprint on the window.
A second example of a prior art system which has attempted to overcome some of the difficulties of fingerprint image capture is disclosed in U.S. Pat. No. 5,096,290 issued to Ohta on Mar. 17, 1992 ("Ohta"). Ohta discloses a fingerprint imaging apparatus which includes a prism having a window against which a fingerprint to be imaged is placed. A layer of transparent material is placed over the window. The layer of material can be natural or synthetic rubbers including silicon and epoxy. When a fingerprint is placed against the material layer, the material tends to deform to accommodate slightly recessed areas of fingerprint ridges placed against the material layer. Thus, the material can act to improve the contrast and sharpness of a fingerprint image.
However, the adhesive with which the material layer is adhered to the surface of the window can contain solvents which can be vaporized around the surface of the layer. And, it has been found that some transparent materials including natural and synthetic rubbers as disclosed by Ohta, can break down and become brittle and or clouded as a result of exposure to these vapors. It has also been found that placing a fingerprint against the pliant material layer can accelerate this breakdown process. As such, the pliant material layer disclosed by Ohta could become brittle with use and, as such, would no longer enhance fingerprint image contrast and sharpness. Additionally, fingerprints can contain oily substances which can leave a residue on the surface of the window which is used to image the fingerprint. Such oily residue can interfere with subsequent capturing of fingerprint images and collect debris which can further interfere with capturing of fingerprint images. Further, as noted above, oily fingerprint substances can leave an image of a fingerprint on the window which can "trick" the fingerprint reader into registering a false match.
As the above discussion makes clear, there is room for improvement in fingerprint imaging devices. Specifically, a fingerprint imaging system should be able to quickly and reliably capture a high contrast, sharp image. The system should also be relatively easy to use and manufacture.